Fix system metrics unit tests on mac

[david-gang]

Description

The system metrics tests on mac are failing:

tox -e py312-test-instrumentation-system-metrics -- -ra

> self.assertEqual(sorted(metric_names), sorted(observer_names)) E AssertionError: Lists differ: ['pro[588 chars]work.dropped_packets', 'system.network.errors'[114 chars]unt'] != ['pro[588 chars]work.connections', 'system.network.dropped_pac[144 chars]unt'] E E First differing element 20: E 'system.network.dropped_packets' E 'system.network.connections' E E Second list contains 1 additional elements. E First extra element 27: E 'system.thread_count' E E Diff is 872 characters long. Set self.maxDiff to None to see it. instrumentation/opentelemetry-instrumentation-system-metrics/tests/test_system_metrics.py:141: AssertionError 

The issue is easy to understand

opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-system-metrics/src/opentelemetry/instrumentation/system_metrics/__init__.py

Line 144 in 109173f

if sys.platform == "darwin":

if sys.platform == "darwin": # see https://github.com/giampaolo/psutil/issues/1219 _DEFAULT_CONFIG.pop("system.network.connections") 

But in the unit tests this metric is still tested.
Fixes # (issue)

Type of change

Skip the check of this metric on mac
Please delete options that are not relevant.

• [V ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Run Unit test on mac and see that everything passes

tox -e py312-test-instrumentation-system-metrics -- -ra

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• [V ] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [ V] Followed the style guidelines of this project
• Changelogs have been updated - No changelog to update
• Unit tests have been added - Just needed to skip one
• Documentation has been updated - Nothing to update. i just skip a fix

[emdneto]

I don't know if we should care about macOS since we don't even support it on CI, but it doesn't hurt to provide this skip for Mac users.